Typically, data is stored on a recording layer of an optical disk by forming either data pits or data marks on the recording layer of the disk. The data pits or marks are formed along servo tracks on the recording layer of the optical disk. A servo track is a permanent physical feature on the recording layer of the optical disk which provides a track-following reference and defines the path along which data is written. Servo tracks may be spiral or concentric. A groove is an example of a servo track. In some types of prerecorded optical disks, such as read only memory (ROM) disks, the data pits formed on the recording layer also function as a servo track.
Typically, an optical transducer which includes a focused laser beam is coupled to a servo track on the recording layer of the optical disk. When reading the optical disk, the data pits or marks formed along the servo track pass by the optical transducer as the optical disk rotates, causing the optical transducer to generate a data signal representing the data stored on the recording layer of the disk. The optical transducer includes a focus positioner and a tracking positioner for maintaining alignment of the focused laser beam with respect to the servo track in the focus direction and the cross-track direction as the optical disk rotates. The focus and tracking positioners include servo-control systems which respond to focus and tracking error signals produced by the optical transducer.
FIGS. 1a, 1b, 1c show a typical mass-produced optical ROM disk in which prerecorded data is stored on an optical disk 10 by forming a predetermined series of data pits 12 along a track 14 of the optical disk 10. FIG. 1a shows a top-view of the optical disk 10. FIG. 1b shows an expanded view of the track 14 shown in FIG. 1a. FIG. 1c shows a cross-sectional view of the track 14 shown in FIG. 1b. The recording layer of the optical disk 10 is permanently formed during manufacturing to create the data pits 12. Therefore, data on an optical disk 10 which is stored by forming data pits 12 on the recording layer of the optical disk 10 can not be erased or re-written.
In a re-writable optical disk, such as a phase change optical disk, data is stored on the recording layer of the optical disk in the form of data marks by controlling the optical characteristics of the recording layer of the disk. Data marks are formed on the recording layer by heating the recording layer of the disk with a focused laser beam at the locations where the data marks are to be written. In phase change recording, the optical reflectivity of the data mark is determined by the crystalline condition of the recording layer. The crystalline condition of the recording layer is determined by controlling the optical power in the focused laser beam. The optical power of the laser beam used to heat the recording layer determines the rate at which the temperature of the recording layer of the optical disk cools where the data mark is located. The rate at which the data mark location of the recording layer cools determines whether the location cools to an amorphous or a crystalline condition. Typically, the recorded data mark is amorphous and the surrounding area is crystalline.
FIGS. 2a, 2b, 2c show a re-writable optical disk 20 in which data is stored on the optical disk 20 by forming a series of data marks 22 along a track 24 of the optical disk 20. FIG. 2a shows a plan-view of the optical disk 20. FIG. 2b shows an expanded view of the track 24 shown in FIG. 2a. FIG. 2c shows a cross-sectional view of the track 24 shown in FIG. 2b. 
In the prior art, placement of data to be written on a recording layer of a re-writable optical disk is typically determined by including synchronization information between fixed-length data fields. A sector is a repeating unit of pre-determined length. FIG. 3a shows a plan-view of a prior art optical disk 30 in which data stored along a servo track 32 is divided into sectors 34. FIG. 3b shows an expanded view of a sector 34 of the optical disk shown in FIG. 3a. The sector 34 includes a header 36, a data field 38 having a predetermined length, and an edit gap 40. FIG. 3c shows an expanded view of the header 36 shown in FIG. 3b. The header 36 includes synchronization information 42 and track address information 44. The synchronization information 42 is also referred to as the sync field. The synchronization information 42 is permanently encoded on the recording layer of the optical disk 30 within the sectors 34. Data written onto the recording layer of the optical disk 30 is synchronized to a write clock. The write clock is synchronized to a clock reference signal which is generated periodically as the synchronization information 42 passes by the optical transducer as the optical disk 30 rotates. The clock reference signal provides position information of the optical transducer with respect to synchronization information 42 on the recording layer of the optical disk 30 when the synchronization information 42 passes by the optical transducer. However, while data within data fields 38 is being written by the optical transducer, the clock reference signal drifts in frequency and phase. That is, when the optical transducer is between points where synchronization information 42 exists, the frequency and phase of the write clock can drift with respect to the synchronization information 42 located within sectors 34. Drift of the write clock with respect to the synchronization information 42 can be caused by disk rotation speed variations, servo track eccentricity and the cumulative effect of other variations in an optical disk recorder such as clock frequency drift. In general, the greater the distance between sync fields, the greater the drift of the write clock.
The edit gap 40 shown in FIG. 3b is included within the sector 34. A data field which includes a fixed number of data bits is typically written to the sector 34 of the recording layer of the optical disk 30. The edit gap 40 accommodates variations in the placement of the last data bit of a data field which is written to the sector 34. That is, although all data fields normally contain the same number of data bits, the edit gap 40 allows the placement of the last data bit of a data field to be different each time the data field is re-written. Therefore, placement of bits written to the recording layer is not required to be as precise as the placement would be required to be if the edit gap 40 did not exist. Edit gaps are needed to accommodate drift of the write clock in prior art re-writable optical disks.
Presently existing DVD read only memory (ROM) formats do not include physical sectoring of data stored on the recording layer of an optical disk. Therefore, synchronization fields and edit gaps are not provided. When reading a ROM optical disk, a read clock is produced from the data stored on the optical disk. Therefore, no synchronization information is required.
The DVD read only memory (ROM) format specification organizes data into fixed-length data fields for error correction code (ECC) purposes. Each data field has an associated header containing synchronization and address information to facilitate data readout. This synchronization and address information is stored on the disk in the form of data pits which are indistinguishable from the data pits used to encode data. Although a DVD-ROM data field, together with its associated header information makes up a “physical sector” for the purposes of a read-only memory, it does not satisfy the requirements of a physical sector for the purposes of a re-writable optical disk memory. For this reason, all sectoring of the DVD format is treated as “logical sectoring.” A logical sector is contained within the data, whereas a physical sector contains the data. Therefore, all synchronization information, addressing and other DVD formatting are treated as if they were data, and are written on the disk in the form of data marks at the same time data is written.
Writing data to the recording layer of a re-writable optical disk which is compatible with DVD-ROM formats therefore requires the data to be written to a disk having no physical sectors on the unwritten disk, and therefore, no address or synchronization information in dedicated areas within the physical sectors. Furthermore, edit gaps can not be included. Without edit gaps, the data marks must be written with sub-bit accuracy during overwriting of pre-existing data.
U.S. Pat. Nos. 4,238,843, 4,363,116, 4,366,564, 4,375,088, 4,972,401 teach methods of permanently providing additional synchronization information along the tracks of an optical disk within data fields. The teachings of these patents also include synchronization information within sync fields between the data fields. Further, the spatial frequency of the synchronization information which is within the data fields must correspond with nulls in the spatial frequency of the data. This requires the data to be encoded using special codes so that nulls in the spatial frequency of the data correspond with the spatial frequency of the synchronization information.
It is desirable to have a re-writable optical disk and an optical disk recorder capable of recording data on the optical disk wherein the recorded disk is compatible with DVD-ROM standard formats, and is readable by a standard DVD reader, and wherein pre-existing data on the optical disk can be re-written (sometimes called over-written) with new data with sub-bit accuracy. The optical disk and optical disk recorder must be capable of generating a write clock which is synchronized with sub-bit accuracy to absolute position along the servo tracks of the optical disk. Further, it is necessary to be able to write using standard DVD data formats.